1. Field of the Invention
This invention lies in the field of rubber articles, particularly those formed by dip-molding. In particular, this invention addresses methods of vulcanization of dip-molded rubber articles.
2. Background of the Invention
Natural rubber latex has been extensively used as a material of construction for elastomeric dip-molded medical devices and medical device components. Examples of medical devices and components made from natural rubber latex are surgical gloves, examination gloves, finger cots, catheter balloons, uterine thermal ablation balloons, catheter cuffs, condoms, contraceptive diaphragms, indwelling urinary drainage catheters, and male external urinary drainage catheters. Other examples will be apparent to those skilled in medicine and in the manufacture and use of these and similar medical devices. Dip-molding techniques are also used in making elastomeric devices non-medical uses. These include toy balloons, industrial gloves, household gloves, and other similar devices. These devices, both medical and non-medical, can also be formed from synthetic rubber latex materials rather than natural rubber. In some cases, synthetic materials are preferred, for example where natural rubber is deemed unsuitable for some reason or where the synthetic material offers an advantage.
In latex dip-molding processes, dip formers are dipped in a latex bath, then withdrawn from the bath, dried in hot air, and vulcanized in hot air. In some cases, the latex is pre-vulcanized, i.e., the rubber particles in the latex are partially or fully vulcanized prior to the dipping step. A prevulcanized latex produces a film with improved wet and dry gel strengths, and when further vulcanization is performed after dipping and hot air drying, the tensile properties are improved. An advantage of prevulcanization is a reduction in the process time by lessening or eliminating the time required for the post-dip vulcanization. In some dip-molding processes, a chemical coagulant is included in the latex or on the dip former, and heat-sensitized coagulant dipping methods are applied to produce articles have a greater film thickness. Multiple dips are also used in some processes. Details of these and other methods are well known to those skilled in the art of latex dip molding. Further descriptions of the process and its variations are found in Pendle, Dipping with Natural Latex, published by The Malaysian Rubber Producers"" Association (1995).
Vulcanization performed on the latex film after the dip former is removed from the bath serves to form covalent bonds both within the individual rubber particles and between coalesced rubber particles. A problem with vulcanization both at this stage and prior to the dip is that the outer surfaces of the particles have greater exposure to the vulcanizing agents than the particle interiors, resulting in a case-hardening effect and a lack of uniformity in the rubber.
In dip-molding processes for rubber latices, sulfur is the primary vulcanizing agent, although various accelerators, activators, sulfur donors, and boosters are frequently included as well. A description of prevulcanization methods and formulations for both natural and synthetic rubber latices is found in Blackley, D.C., Polymer Latices: Science and Technology, 2d Edition, Vol. 2, Chapter 13 (Chapman and Hall, 1997). Prevulcanization methods performed without sulfur are those utilizing free radical crosslinking, which can be achieved by various means, including high energy irradiation in the presence of a chemical sensitizer. Natural latex prevulcanized in this manner is referred to as xe2x80x9cradiation vulcanized natural rubber latexxe2x80x9d (RVNRL). Descriptions of such latices and the vulcanization processes used in their preparation are found in Zin, W.M.B.W., xe2x80x9cSemi industrial scale RVNRL preparation, products manufacturing and properties,xe2x80x9d Radiat. Phys. Chem., 52(1-6), pp. 611-616 (1998).
Rubber films from RVNRL are produced by simply casting the latex into films and then drying the films. No vulcanization is done after the film is cast, and none can be done unless curative agents are subsequently added. Films made by this process have tensile strengths of up to 27.1 megapascals (3930 psi). While this meets the requirements of many dip-molded rubber devices, such as surgical gloves for example, the tensile strength of these films is not as high as that achieved in many sulfur-vulcanized films where a post-vulcanization step (after the dip stage) is included. The RVNRL films are also lower in the value of the 100% tensile modulus than sulfur-vulcanized films. The RVNRL films also suffer from a lack of any means to achieve true particle integration by covalent bonds. This makes it difficult to form a truly integrated, pore-free latex rubber film from RVNRL. A further disadvantage is the need for access to an irradiation facility, which may not be in a location that is convenient to many rubber manufacturers and which adds considerably to the cost of manufacture.
An alternative means of prevulcanization of latex by free radical crosslinking is that which involves the use of organic peroxides and hydroperoxides. Latex that is prevulcanized with these materials is referred to as xe2x80x9cperoxide vulcanized natural rubber latexxe2x80x9d (PVNRL). Descriptions of such latices and methods for preparing them are found in U.S. Pat. No. 2,868,859, issued Jan. 13, 1959, to G. Stott, entitled xe2x80x9cCuring Natural Rubber Latex With a Peroxide.xe2x80x9d The process disclosed in this patent involves superheating natural rubber latex in the presence of 2% (based on dry rubber weight) ditertiary butyl peroxide in a pressure vessel at a temperature of 170xc2x0 C. for fifteen minutes. The latex was then cooled, and the films cast and dried to yield vulcanized rubber films with a tensile strength as high as 251 kg/cm2 (3739 psi). The film was formed simply by drying, with no post-drying vulcanization. Unfortunately, utilization of this process on a commercial scale would require large and expensive heated pressure vessels, and prevulcanization is a necessary part of the process.
Latex prevulcanized with a hydroperoxide rather than an organic peroxide is described in U.S. Pat. No. 2,975,151, issued Mar. 14, 1961, to W. S. Ropp, entitled xe2x80x9cVulcanization of Latex With Organic Hydroperoxide.xe2x80x9d In this patent, natural rubber latex is prevulcanized by superheating under pressure at 250xc2x0 F. (121xc2x0 C.) for about one hour with cumene hydroperoxide. The resulting cooled latex is cast into a film, then air dried. The product had a maximum tensile strength of 2775 psi. As in the Stott patent, the utilization of this process on a commercial scale would require large scale heated pressure vessels, and the tensile strength is not nearly as good as that of a sulfur-vulcanized latex or of the organic peroxide prevulcanized latex of Stott.
The use of hydrogen peroxide as a prevulcanizing agent with an activating chemical is disclosed in U.S. Pat. No. 3,755,232, issued Aug. 28, 1973, to B. K. Rodaway, entitled xe2x80x9cVulcanization of Latex With Organic Hydroperoxide.xe2x80x9d The method of this patent is performed at lower temperatures without the use of pressure vessels. The patent cites an example in which a natural rubber latex is prevulcanized by this method, cast into a film and dried, to yield a product with a tensile strength of 124 kg/cm2 (1760 psi). Thus, despite its advantages this process produces latex films of interior strength. The possibility of adding a sulfur curative system to the latex after prevulcanization to permit post-casting vulcanization is suggested, but this would involve the use of sulfur curative chemicals, which peroxide processes are generally intended to avoid. In further examples, curing of polychloroprene and other synthetic latices is performed with hydrogen peroxide and an activator, the products in each case having inferior tensile properties.
Further disclosure of technology forming the background of the present invention is found in U.S. Pat. No. 3,892,697, issued Jul. 1, 1975, to O. W. Burke, entitled xe2x80x9cPreparation of Crosslinked Polymer Latex From Aqueous Emulsion of Solvent/Polymer Solution of Precursor Latex Particle Size.xe2x80x9d In the process disclosed in this patent, dicumyl peroxide is mixed with a synthetic polyisoprene latex under 6000 psi pressure, and the mixture is subjected to an unspecified elevated temperature for an unstated period of time. There is no disclosure of film formation.
Still further methods forming the background of the invention are those known as xe2x80x9ccontinuous vulcanization in liquid bathsxe2x80x9d (LCM Vulcanization) which are used on extruded rubber profiles. In LCM Vulcanization, a solid constant profile shape is extruded, then submerged in a hot liquid bath such as molten salt, hot oil, or melted lead, or in a hot fluid medium such as fluidized sand particles. Essentially all molecular oxygen is excluded from the curing environment. The use of the hot liquid bath or fluid medium is to provide very rapid heat transfer rates to thin-wall extruded rubber profiles. Descriptions of various LCM Vulcanization methods are found in Hoffman, Rubber Technology Handbook, pages 394-398 (Hanser Publishers, 1994), and in U.S. Pat. No. 4,981,637, issued Jan. 1, 1991, to M. L. Hyer, entitled xe2x80x9cMethod of Forming an Improved Wiper Blade.xe2x80x9d These references do not disclose application of the process to dipped films.
Latex articles formed by dip molding must be pore-free if the passage of pathogens or other unwanted substances through the article walls is to be prevented. Pore-free walls require good integration and adhesion between the rubber particles of the latex. Many attempts have been made to achieve this, but it remains a difficult goal. Excessive vulcanization for example tends to inhibit particle integration. A simple means of determining the extent of prevulcanization is a test known as the chloroform coagulation test. A description of this test can be found in The Vanderbilt Latex Handbook, 3d Edition, page 110 (R.T. Vanderbilt Company, Inc., Norwalk, Conn., USA).
All patents and publications cited in this specification are incorporated herein by reference.
It has now been discovered that pore-free rubber articles can be prepared by dip-molding processes by including vulcanizing agent(s) in the dipping medium and vulcanizing the wet film by immersing the dip former in a heated liquid bath that is chemically inert. The temperature of the heated bath will be sufficiently high to cause at least a partial melting and/or softening of any coalesced rubber particles in the film while vulcanizing the film, and the time needed to effect vulcanization under these conditions is considerably less than that typically used for vulcanization in hot air. The resulting film is coherent and essentially pore-free.
Another aspect of this invention resides in the discovery that the tensile properties of an article of vulcanized rubber can be improved to an unusually effective degree by immersing the already vulcanized article in a solution of a vulcanizing agent to cause the rubber of the article to absorb or imbibe the vulcanizing agent from the solution, and then immersing the rubber and the imbibed vulcanizing agent in a heated liquid bath that is substantially free of molecular oxygen and chemically inert. After recovery of the article from the bath, the tensile properties are considerably greater than those that the product would have if the same amount and type of vulcanizing agent were included in the original vulcanization.
This invention is useful in the manufacture of articles of all rubber materials, both natural and synthetic. For certain aspects of this invention, notably those that reside in the use of the heated liquid bath for a single-stage vulcanization after dip-molding, the preferred rubber materials are those other than cis-1,4-polyisoprene.
Among the many advantages of this invention is a faster vulcanization rate without the risk of undesirable oxidation of the dipped parts. The invention also offers superior particle integration and thus more coherent latex films by partially melting the particles as they are being crosslinked and heating them more thoroughly, which reduces the tendency of the particles toward case hardening. Prevulcanization, i.e., vulcanization performed on the dipping liquid prior to the dip stage, can be eliminated in many cases, and this offers advantages for latices that are peroxide cure systems or sulfur cure systems where prevulcanization is used in part to reduce the cure times and to reduce the quantity of nitrosamines that may be released during dip molding operations. Alternatively, the postvulcanization of the invention, referring to its occurrence subsequent to the dip stage, can improve the tensile properties of the product without the need for the addition of sulfur-based chemicals. Postvulcanization can also be performed using a different reaction than that used for the prevulcanization. For example, postvulcanization with the use of peroxides can be performed on latices that are prevulcanized by sulfur, by peroxide, or by radiation. In systems that are susceptible to nitrosamine formation, the invention reduces or eliminates the amount of nitrosamines that are formed. When polychloroprene latices, nitrile latices or mixtures of the two are used, postvulcanization by use of organic peroxides can be achieved with very small amounts of the peroxides. With peroxide-based and radiation-based vulcanization systems, the use of the present invention provides products with a 100% tensile modulus that is higher than has been previously obtained with such systems, and yet with no loss of tensile strength. With sulfur-based systems, the high-temperature, oxygen-free environment helps to prevent the degradation that is caused by hydroperoxides that are typically generated during hot air vulcanization. Such degradation is responsible in part for the aging of latex. Use of the invention in non-sulfur-containing systems such as peroxide-based systems results in products with a longer shelf life. Still further, the elimination of the need for hot air curing and its inherent inefficiencies offers considerable savings in energy, since hot media baths are easily insulated. Further energy savings are also available when prevulcanization and maturation are eliminated. The means by which these and other objects and advantages are achieved, as well as particulars of the process and its preferred embodiments, will be evident from the description that follows.
The liquid bath in which the dip former and film are immersed subsequent to the dip stage of the process is a heated liquid that provides rapid heat transfer to the film. Further properties of liquid media that are most desirable and therefore preferred for this purpose are the lack of a tendency to migrate or diffuse into the film on the dip former (unless the medium itself is a desirable constituent of the film), the quality of being stable with respect to the surrounding environment (both the atmospheric environment and the rubber-forming material as well as the various species that may be compounded with the material), and the quality of remaining liquid at the vulcanization temperature. Examples of liquid media that can be used for this purpose are molten inorganic salts, oils, glycols, liquified metals, water, and brine solutions. Preferred among these are molten inorganic salts, silicone oils, and glycols, and the most preferred are molten inorganic salts. Examples of suitable molten inorganic salts are nitrates, nitrites, carbonates, sulfates, phosphates, and halides of potassium, sodium and lithium, as well as combinations of salts of this group. Salt combinations of this type are commercially available from such suppliers as Heatbath Corporation, Detroit, Mich., USA; and Hubbard-Hall Inc., Inman, S.C., USA. An example of a suitable commercial salt mixture is QUICK CURE 275 of Hubbard-Hall, Inc., the main components of which are potassium nitrate (approximately 50% by weight), sodium nitrite (approximately 30% by weight), and sodium nitrate (less than 10% by weight), with a molten temperature range of about 315xc2x0 F. to 650xc2x0 F. (157xc2x0 C. to 343xc2x0 C.). Other examples are PARCURE 275 and PARCURE 300 of Heatbath Corporation.
The heated liquid medium bath is preferably used at a temperature that significantly exceeds the temperatures used in hot air vulcanization methods of the prior art, but not so high as to have an adverse effect on the stability of the rubber being vulcanized. When the rubber is natural rubber, for example, it is best not to exceed 450xc2x0 F. (232xc2x0 C.), and in the case of styrene butadiene rubber or polychloroprene latex, it is best not to exceed 575xc2x0 F. (302xc2x0 C.). A preferred temperature range for the full scope of this invention is about 100xc2x0 C. to about 350xc2x0 C. For polychloroprene and styrene-butadiene rubber, a preferred temperature range is about 150xc2x0 C. to about 300xc2x0 C., while for natural rubber a preferred temperature range is from about 150xc2x0 C. to about 235xc2x0 C. The choice of operating temperature and exposure time will be subject to considerations both of achieving a rapid cure and of maintaining an economic use of energy and other process costs. Other considerations may be present with particular types of rubber and particular curing systems. In organic peroxide curing systems, for example, the preferred temperature and time will be those that result in cleavage of essentially all of the peroxide present. This is generally achieved in six to eight half-lives. In sulfur-based curing systems, the avoidance of reversion and toxicity are often considerations. In all cases, however, the time necessary for full curing is much less than that required in hot air curing processes of the prior art. A presently preferred cure condition is nine minutes at 350xc2x0 F. (177xc2x0 C.).
This invention is applicable to a wide range of rubber and rubber substitute compositions, including both latices and organic solutions.
Of the latices, the one most commonly used is natural rubber. Natural rubber can be obtained from several sources, including Hevea brasiliensis, Parthenum argentatum (commonly known as xe2x80x9cguayulexe2x80x9d), and Ficus elastica rubber trees. Methods for obtaining natural rubber latices from non-Hevea sources are described in U.S. Pat. No. 5,580,942, issued Dec. 3, 1996 to Cornish (xe2x80x9cHypoallergenic Natural Rubber Products From Parthenum Argentatum (Gray) and Other Non-Hevea Brasiliensis Speciesxe2x80x9d). Natural rubber latex is available in several grades, including high ammonia latex, low ammonia latex, and others. All such varieties are suitable for use in the present invention. This invention also extends to natural rubber latices that have been processed to reduce the amount of proteins present in the latices. Some of these processes include centrifuging to separate and remove water, and others include double centrifuging, in which an initial centrifuging is followed by the addition of water and a second centrifuging. Still other processes involve the use of enzymes to digest the proteins. Descriptions of enzyme methods are found in U.S. Pat. Nos. 5,610,212 (xe2x80x9cMeans for Mechanically Stabilizing Deproteinized Natural Rubber Latex,xe2x80x9d Mar. 11, 1997), 5,569,740 (xe2x80x9cDeproteinized Natural Rubber Latex and Its Production Process,xe2x80x9d Oct. 29, 1996), and 5,585,459 (xe2x80x9cProcess for Producing Raw Rubber,xe2x80x9d Dec. 17, 1996), to Tanaka et al. An example of a commercially available deproteinized rubber latex is ALLOTEX, obtainable from Tillotson Healthcare Corporation, Rochester, N.H., USA.
Synthetic rubber latices in general are likewise usable in the practice of this invention. Examples are ethylene-propylene-diene terpolymer, styrene isoprene rubber, styrene butadiene rubber, styrene isoprene butadiene rubber, polybutadiene rubber, polychloroprene, nitrile rubber, styrene block copolymers, and butyl rubber. An example of a polychloroprene latex is NEOPRENE 750, available from E.I. DuPont de Nemours, Inc. Wilmington, Del., USA, and an example of a nitrile latex is NITRILE LATEX #O17071, available from Heveatex Corporation, Fall River, Mass., USA. Mixtures of latices can also be used. Some of these mixtures are described in U.S. Pat. No. 3,626,052, issued Dec. 7, 1971, to Sisco et al., entitled xe2x80x9cPolyisoprene-Neoprene Meteorological Balloons,xe2x80x9d where polychloroprene latex is mixed with polyisoprene latex to produce meteorological balloons.
This invention also extends to polymer dispersions that are used in a manner similar to rubber latices. One example is an aqueous dispersion of a polyurethane thermoplastic elastomer. A commercially available dispersion of this type is INTACTA, available from The Dow Chemical Company, Midland, Mich., USA. Polymer dispersions such as this lack carbon-carbon double bonds and hence are not susceptible to sulfur-based crosslinking. For these dispersions, embodiments of the present invention that use curing systems other than those that are sulfur-based can be used. Polyurethane products such as medical examination gloves that are formed by the process of this invention exhibit increased resistance to solvents.
In addition to latices and polymer dispersions, the present invention also applies to organic solutions. The organic solvents used in forming these solutions are any solvents that are inert to the rubber, rubber substitute or polymer, and that are readily removable from the dip-molded film by evaporation. The solvent is preferably an aliphatic hydrocarbon, saturated or unsaturated, linear, branched or cyclic, or ethers, esters, alcohols or amines. Typical solvents are aliphatic hydrocarbons containing 5 to 8 carbon atoms, such as pentane, pentene, hexane, heptane, cyclohexane, and cyclopentane, and heterocyclic compounds such as tetrahydrofuran.
A wide variety of vulcanizing agents can be used in the practice of this invention. Useful vulcanizing agents include organic peroxides, sulfur-containing compounds, selenium-containing compounds, and tellurium-containing compounds. Organic peroxides, for example, may be used singly or in combination, and the most common types are dialkyl peroxides, peroxyketals, and dialkyl peroxides. Preferred organic peroxides are the dialkyl peroxides, particularly dicumyl peroxide, available from Hercules Incorporated, Wilmington, Del., USA, as DICUP R. Other useful dialkyl peroxides are 2,5-dimethyl-di-(t-butylperoxy)hexane, di-t-butylperoxide, t-butylcumyl-peroxide, bis(t-butylperoxyisopropyl)benzene, butyl4,4-bis(t-butylperoxy)valerate, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane, 2,5-bis(t-butylperoxy)-2,5-dimethyl-3-hexyne, t-butyl 3-isopropenylcumyl peroxide, bis(3-isopropenylcumyl) peroxide, 1,1 -bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, t-butylperoxybenzoate, and bis(2,4-dichlorobenzoyl) peroxide.
Coagents and other additives are often used in conjunction with the organic peroxides to achieve products having particular properties. Certain coagents also add to the crosslinking efficiency of the peroxides by causing a single peroxide radical to produce more than one carbon-carbon crosslink. Coagents can also be integrated into the polymer network by covalent bonds to enhance certain properties of the polymer, such as elongation and tear strength. Some of these coagents are based on acrylate and methacrylate chemistry. All however are suitable for inclusion in the methods and products of the present invention. Examples of suitable coagents are SARET 516, SARET 517, SARET 521, and SARET 634, available from Sartomer Company, Inc., Exton, Pa., USA. These coagents are multifunctional salts of acrylic and methacrylic acids. Of this group of coagents, SARET 634 (whose primary ingredient is zinc dimethacrylate) and SARET 521 (whose primary ingredients are difunctional acrylate esters) are the most preferred. Trimethylolpropane trimethacrylate another example. A more extensive description of such coagents is found in U.S. Pat. No. 3,751,878, issued Aug. 7, 1973 to Cowperthwaite et al., entitled xe2x80x9cInhibiting Prevulcanization of Rubber With Polyfunctional Methacrylate Monomers as Cross-Linking Coagents with Peroxides,xe2x80x9d and U.S. Pat. No. 5,310,811, issued May 10, 1994 to Cottman et al., entitled xe2x80x9cFree Radical Cured Rubber Employing Acrylate or Methacrylate Esters of Hydroxybenzene and Hydroxynaphthalene Compounds as Co-Curing Agents.xe2x80x9d
Free-radical vulcanizing agents other than peroxides are disclosed in U.S. Pat. No. 3,892,697, referenced above.
Sulfur-based vulcanization systems include both small sulfur-containing molecules and sulfur-containing polymers. Examples of sulfur-based vulcanization chemicals are:
mercaptothiazoles, for example 2-mercaptobenzothiazole and its salts, notably its zinc salt
thiuram sulfides and disulfides, for example tetraethylthiuram monosulfide, tetrabutylthiuram monosulfide, tetramethylthiuram disulfide, and tetraethylthiuram disulfide,
guanidines
thiourea and substituted thioureas
thiocarbanilides and substituted thiocarbanilides, for example o-dimethyl-thiocarbanilide and its isomers and alkyl homologs
zinc alkyl dithiocarbamates, for example zinc dimethyl dithiocarbamate, and accelerators containing these materials
sodium or potassium dimethyl dithiocarbamate
selenium dialkyl dithiocarbamates, for example selenium diethyldithiocarbamate
2-benzothiazyl-N,N-diethylthiocarbamyl sulfide
xanthates such as dibutyl xanthogen disulfide and xanthogen polysulfide
alkyl phenol sulfides
dipentamethylene tetrasulfide
sulfur-containing polymers such as Thiokol VA-3
4,4-dithiomorpholine
miscellaneous disulfides such as bensothiazyl disulfide and bis(dimethylthiocarbamoyl) disulfide
When the dip-molded articles of this invention are intended for use in contact with human skin, the preferred compounding ingredients are those that produce films that are biocompatible. Examples of compounding ingredients that serve this purpose for sulfur-vulcanized systems are xanthogen compounds such as diisopropyl xanthogen polysulfide, dibenzyldithiocarbamate, and higher alkyl zinc dithiocarbamates. For peroxide vulcanized systems, the preferred compounding ingredient is dicumyl peroxide.
Reinforcing agents and other additives are also included in some embodiments of the invention. Examples of reinforcing agents are fumed silica, carbon black, and chopped fibers. The use of cut fibers for example to improve the tear strength of medical gloves is disclosed in U.S. Pat. No. 6,021,524, issued Feb. 8, 2000, to Wu el al., entitled xe2x80x9cCut Resistant Polymeric Films,xe2x80x9d and the use of fumed silica to improve the tear strength of dipped films is disclosed in U.S. Pat. No. 5,872,173, issued Feb. 16, 1999, to Anand, entitled xe2x80x9cSynthetic Latex Compositions and Articles Produced Therefrom.xe2x80x9d Antioxidants and antiozonants may also be included to protect against environmental aging. Pigments and dyes may also be included, as may any of the other additives known to those skilled in the art of the formulation and manufacture of rubber devices.
An illustrative procedure for latex dip molding and curing in accordance with the present invention is as follows:
1. Either a natural rubber or a synthetic rubber latex is compounded with vulcanizing agent(s) and possibly an antioxidant, a stabilizer or both. If organic peroxide vulcanization is used, it will often be sufficient to simply add to the latex a dispersion that contains an organic peroxide.
2. Prevulcanization of the latex at this stage is optional and not required for all embodiments of this invention. When used, prevulcanization can improve the wet gel strength.
3. A dip former is optionally coated with a chemical coagulant by dipping the former into a bath of a coagulant-containing liquid, then withdrawing the former and drying it.
4. The dip former, with or without the coagulant coating, is dipped in a bath filled with the compounded latex.
5. The dip former is slowly withdrawn from the bath. If the former had a coagulant coating, it now has a wet latex gel on its surface. If no coagulant coating was applied, the former will have a liquid latex film on its surface.
6. Excess water in the latex film on the dip former surface is removed, generally by evaporation in a hot air convection oven with either sweep gas or a partial vacuum. The process can be supplemented with infrared, microwave, or radiofrequency radiation, or any other type of energy to expedite the evaporation. Vacuum drying is advantageous since it avoids the need for exposure of the dried latex to air at an elevated temperature prior to vulcanization.
7. The latex is cured by immersion of the dip former into the heated liquid media bath for sufficient time to cure the latex.
8. The dip former with the cured latex film is withdrawn from the heated medium bath and cooled either in air or in a stream of water. Water may be used to rinse off any excess solidified heat transfer medium such as solidified salt.
9. The vulcanized latex article is manually or mechanically stripped from the dip former.
An illustrative procedure for solvent dip molding and curing in accordance with the present invention is as follows:
1. Solid granules of synthetic or natural rubber elastomer are dissolved in a suitable solvent to form a cement. Suitable compounding agents are dispersed or dissolved in the cement. Compounding agents similar to those used in the latex processes, including organic peroxides, can be used.
2. No prevulcanization is necessary, as all compounding gents are uniformly dispersed in the cement. The cement is placed in a dip tank, and a dip former is dipped in the cement.
3. The dip former is slowly withdrawn from the dip tank to leave a film of the cement over the surface of the dip former.
4. Solvent is evaporated from the dip former to leave a uniform polymer film on the surface. Removal of the solvent can be achieved by ambient or hot air drying.
5. The polymer film is cured by immersion of the dip former in a heated liquid medium bath. After a suitable period of time, the dip former is withdrawn from the bath and cooled in air or a stream of water.
6. The dip former is then soaked in water to help break the adhesion between the film and the dip former.
7. The vulcanized latex article is manually or mechanically stripped from the dip former.
While the present invention virtually eliminates the need for prevulcanization and maturation of the compounded latex or solution, prevulcanization is useful with latices that would otherwise have an exceptionally low wet or dry gel strength. Prevulcanization can be done by any conventional method. Such methods include, but are not limited to, sulfur prevulcanization, peroxide prevulcanization, and prevulcanization by high energy irradiation, all of which may be performed as they are in the prior art. Good wet gel strength is useful in preventing cracks from forming in the film as the film is being dried. In the case of natural rubber, both wet and dry gel strengths are generally adequate without prevulcanization. The gel strengths of some synthetic latices are lower, however, and prevulcanization may improve the processing, but is not essential. Prevulcanization by high energy irradiation can also serve to reduce the amount of vulcanization chemicals needed and hence the levels of undesirable residual chemicals in the final product.
It is often useful to determine the extent to which a dipped film or article has been vulcanized. A commonly used method is to cut out a circular disk of the cured film and measure the change in diameter upon immersion of the disk in a solvent bath. A detailed explanation of this test and its use with polyisoprene latex is found in U.S. Pat. No. 3,215,649, issued Nov. 2, 1965, to Preiss et al., entitled xe2x80x9cSynthetic Latex.xe2x80x9d Similar test methods are available for other types of vulcanized polymers, and are well known to those skilled in the art.
After the dip-molded part is vulcanized, further vulcanization can be performed as an optional means of further improving the properties of the product. A preferred method is to imbibe the vulcanized film with a further amount of vulcanizing agent(s), followed by a second beat treatment in a hot liquid bath. The vulcanizing agent may be the same or different than that used in the first stage (immediately following the dip-molding stage). Likewise, the hot liquid bath may be the same or different than that used earlier.
For films vulcanized with dicumyl peroxide, for example, the cured rubber film can be immersed in a solution of dicumyl peroxide in a solvent such as n-pentane, n-hexane, toluene, or ethyl acetate. The peroxide solution significantly swells the film, thereby causing the dicumyl peroxide and solvent to uniformly penetrate the cured film. The film is then withdrawn from the solution and the solvent evaporated, leaving a predictable amount of dicumyl peroxide in the film. The film is then immersed in a hot liquid bath as before for an appropriate period of time, which may be the same period of time used after the initial dip in the latex. The film is then removed and rinsed in water. Other vulcanizing agents or combinations of vulcanizing agents can be substituted to similar effect.
The physical properties of crosslinked articles that are vulcanized in this two-step postvulcanization process are different from, and frequently better than, those of crosslinked articles in which only a single postvulcanization has been performed. This second postvulcanization thus permits a reworking of or an enhancement of the properties of films that have already been vulcanized. This is particularly useful, for example, in the case of right-heart catheter balloons, where the second postvulcanization can achieve significantly higher levels of air inflation and burst pressures. Returning to the dicumyl peroxide example, a typical range of dicumyl peroxide for a high quality right heart catheter is about 1 to about 1.5 phr (parts hundred ratio, or parts per hundred weight of dry rubber). Of this, 0.2 to 0.5 extra phr of dicumyl peroxide can be imbibed with a subsequent heating step to achieve a significant improvement in the air inflation and burst properties.